1. Field of the Invention
The subject invention relates to superconductive materials and wave transmission devices. More particularly, the invention relates to testing the microwave frequency impedance and transition temperature of a superconductive material by use of a resonant cavity and mechanically switched waveguides.
2.Description of the Prior Art
Superconducting materials have great potential for a variety of microwave frequency applications; however, the design of practical devices for these applications is presently severely limited by the lack of knowledge of the electrical characteristics of these materials at microwave frequencies even though the direct current characteristics may be well-known. For example, these materials, which by definition have substantially no DC resistivity, have significant losses at microwave frequencies. This lack of knowledge includes such basic information as the losses at different temperatures in a range thereof involving the transition temperature of the material to the superconducting state and the effect of varying microwave frequencies on the losses. This last effect is particularly important since the losses in a superconductor vary with the square of the frequency instead of only with the square root of the frequency as in non-superconducting metals commonly used for waveguides, antenna elements, and the like.
While the resistivity of a material at microwave frequencies is not directly measurable, there are well-known techniques fully effective at the usual environmental temperatures for deriving this resistivity from measurements of the amplitude and phase of microwave energy as affected by transmission through, reflection from, and resonance within microwave elements having an interior surface, or surface portion, of the material of interest. It is apparent that such measurements are, necessarily, carried out using waveguides and coaxial transmission lines to connect the measurement equipment, such as a well-known microwave network analyzer, to an element including such material. It is thus necessary to calibrate the measuring equipment, including any such transmission elements, by waveguides or the like of known characteristics to isolate the effects due to an element of interest from effects due to the measurement equipment itself. If the measurement equipment, transmission devices, and measured element are all at the same temperature such calibration can be effectively carried out to mathematically derive "error boxes" combining the effects of the measuring and transmission equipment. However, it is apparent that if this equipment varies in temperature, either with time or along the length of transmission elements, the temperature caused changes in length and microwave characteristics of the elements make calibration highly inaccurate if not problematical. It is also necessary that measurements be carried out shortly after calibration to minimize "drift" of the measuring equipment. It is apparent that, for meaningful calibration, any required changing of microwave connections between different calibration and test devices must occur with minimal changes in transmission element length and, to avoid changes in microwave reflection characteristics, with minimal bending of transmission elements and breaking of connections.
These difficulties in measurement and calibration are particularly serious in measurements of the microwave characteristics of superconducting materials where the measurements are necessarily carried out at cryogenic temperatures, commonly defined as below -50.degree. C., and typically involving superconductivity transition temperatures of 90.degree. to 150.degree. K. Insofar as known to the applicants, such measurements have hitherto been carried out by immersing a body, which defines a microwave cavity having an interior surface portion constructed of the material of interest, into liquified cryogenic gas in an open cryostat to bring such portion to the transition range, the measurements then being made with microwave connections to the body made by relatively long transmission elements extending through the liquified gas and from the body and cryostat to microwave measurement equipment at room temperature. This arrangement has a number of deficiencies.
The most serious such deficiency is due to the transmission element lengths and characteristics varying with the temperature difference along the elements so that calibration to eliminate the effects of these elements, which involves measurement of phase relations between microwave energy provided to and returned from these elements and the cavity, is not possible even if these effects were unchanging with time. It is thus not possible in the prior art arrangements to separate corresponding amplitude changes in such energy due to the superconducting material from those due to the transmission elements. A most important measurement of the characteristics of a superconductive material involves changing its temperature, as by an electric resistance heater in the cryostat and applied to the body, to determine the transition point. In such prior art arrangements this measurement itself induces time varying temperature changes in the transmission elements making it even more difficult to separate effects in the measurement equipment from those actually of interest.
Another deficiency of the prior art arrangements arises from the time required, typically 4 to 8 hours, for the transmission and cavity defining elements to cool from room temperature and stabilize sufficiently to permit even the relatively imprecise measurements hitherto possible. Measurements with these arrangements are thus very time consuming, especially when it is desired to change the cavity or superconducting element therein. This lengthy stabilization time also allows the measurement equipment to drift after calibration with elements at room temperature, further reducing the accuracy of the measurements at cryogenic temperature.